We have recently shown that mature and immature B lymphocytes actively replicate upon transfer into B-cell deficient hosts (Cabatingan et.al. J. Immunol. 2002. 169:6765). This homeostatic B cell proliferation (B cell HP) is dependent on Bruton's tyrosine kinase (BTK), inhibited at high B cell densities, independent of antigen and T cells, active at low levels in normal hosts and distinct from more "conventional" mitogenic responses as replicating cells retain a naive phenotype. New results show B cell HP absolutely requires BLyS (B lymphocyte stimulator) as a costimulator that induces the biochemical changes necessary to support B cell growth and division. BLyS stimulation is BTK-independent and synergizes with BTK-dependent inductive signals to initiate B cell HP. HP in B cells is a unique response to B cell deficit that may be important for maintaining B cell numbers and selectively expanding the naive B cell repertoire. The proposed studies will identify unique components of each of the signal pathways necessary for B cell HP, establish the interdependence of the BCR, the BLyS receptor (BR3) and B cell HP and determine the anatomical sites and cells necessary to support B cell HP. Aim 1 is to determine the mechanism of HP signaling using pharmacological inhibitors to define critical components unique to the BTK-dependent inductive pathway and the BLyS dependent BTK-independent lymphotrophic pathway using separate in vitro assays specific for each process. The functional relevance of the signaling components initially identified in this screen will be confirmed using genetic approaches with in vitro and in vivo analyses. Aim 2 is to determine the interactions between BCR and BR3, the two major nonredundant regulators of B cell homeostasis and B cell HP. Using BCR ablated and monoclonal B cell populations, we will determine if the BCR modulates HP by affectingBR3 expression or function and determine directly if the BCR can provide the BTK-dependent inductive signals required for HP thereby biasing the naive Ig repertoire. Aim 3 is to identify the location and cells in the spleen that deliver the inductive and lymphotrophic signals necessary for B cell HP and to determine if ligands delivering these signals are unique to the spleen. Understanding the mechanism of B cell HP will provide new strategies for effecting immune reconstitution in the aged or immunodeficient or following chemotherapy or recovery from lymphotoxic infections.